[llvm-readobj] Change errors to warnings for symbol section name dumping

Also only print each such warning once.

LLVM-style output will now print "<?>" for sections it cannot identify,
e.g. because the section index is invalid. GNU output continues to print
the raw index. In both cases where the st_shndx value is SHN_XINDEX and
the index cannot be looked up in the SHT_SYMTAB_SHNDX section (e.g.
because it is missing), the symbol is printed like other symbols with
st_shndx >= SHN_LORESERVE.

Reviewed by: grimar, MaskRay

Differential Revision: https://reviews.llvm.org/D69671